This invention relates to a thermal cut-off fuse which uses no mechanically operable part, enjoys simplicity of construction and high accuracy of operation, and serves in an electric appliance incorporating a heat source the purpose of breaking the electric circuit of the electric appliance when the ambient temperature of the electric appliance is caused, for some reason or other, to rise from the rated range of service temperatures and reach a dangerous temperature zone and consequently stopping the phenomenon of heat generation.
A thermal cut-off fuse of one type is designed so that a thermal pellet capable of assuming a solid state at normal temperatures below a fixed level and a liquid state at or above the fixed level of temperature is disposed inside the housing of the fuse and, upon elevation of the ambient to the fixed level, the thermal pellet is transformed from the solid state to the liquid state is consequently diminished in volume and this voluminal change of the pellet is converted into a mechanical movement of relevant internal elements of the fuse to open the contact mechanically and break the electric continuity between the electrodes.
Various thermal cut-off fuses have heretofore been developed on the basis of this operating principle. They are invariably provided with contact means braced up in position by some proper resilient means and auxiliary means adapted to actuate freely this contact means as desired. Thus, they inevitably have a complicated construction and entail a heavy burden in terms of design, fabrication and cost. Furthermore, since they use means for producing mechanical movements, they tend to develop mechanical troubles and consequently pose problems of inferior reliability.
On the other hand, there has been developed a thermal cut-off fuse which uses no mechanically operable parts and serves a mere purpose of breaking the electric circuit in which the fuse is contained when the ambient temperature rises excessively. A typical thermal cut-off fuse of this type is characterized by using a fusible alloy as the material for the active element adapted to break the electric circuit at the fixed level of temperature. Such a fusible alloy is not melted completely at one point of temperature but melts over a fairly wide range of temperatures. When the alloy is kept at an elevated temperature near its melting point for a long time, the melting point is greatly varied. In the use of such a conventional thermal fuse, therefore, it has been found necessary to select and use a fusible alloy whose melting point is considerably lower than the upper limit of allowable service temperatures rated for the electric appliance. Incorporation of such a fusible alloy in effect lowers the efficiency of the electric appliance with a heat source to an extent more than is actually necessary.
An object of this invention is to provide a thermal cut-off fuse which has no mechanically operable element and enjoys simplicity of construction and which, upon insertion in the electric circuit of an electric appliance provided with a heat source, enables the electric continuity of the circuit to be broken precisely at the time that the ambient temperature reaches the fixed level.